This invention relates to the art of induction heating and, more particularly, to an improved method and apparatus for inductively heating and hardening surfaces having a non-circular profile relative to the workpiece axis of rotation and/or having a non-uniform mass geometry adjacent to the heated area of the workpiece.
The present invention finds particular utility in connection with the heating and hardening of diesel cam shaft lobes and, accordingly, will be disclosed in detail in connection with such use.
At the same time, it will be appreciated that the invention is applicable to the induction heating of non-circular surface profiles and/or adjacent areas of non-uniform mass geometry in workpieces other than diesel or other cam shaft lobes.
The selective hardening of the non-circular profiles of the cam lobes for heavy duty diesel engine applications presents major challenges with respect to producing a reasonable, acceptable, uniform depth of hardening to provide a highly durable, long wearing surface which, in use, can be subjected to high applied hertzian operational stress. The mass geometry configurations of these cam shafts can be substantially complex in that the cam profiles are varied and include both positive and negative ramp profiles modified for specialized operating requirements. In addition, there are special applications such as the integral ejector cam on a diesel cam shaft which can have a different set of cam profile variables and, in use, have higher hertzian stress levels and a higher engaged surface contact velocity. The mass geometry arrangements are further complicated by the diameter of the basic shaft section between adjacent cams which can substantially change the thermal mass inertia around the circumference of the cam surface, particularly when the diameter of the basic shaft literally matches the cam surface at the heel of the cam.
To rotate a cam of the foregoing character about the axis of an encircling inductor does not provide for obtaining an inductive heating of the outer surface to a predetermined depth of heating and, accordingly, in connection with quenching of the heated surface, does not enable achieving a uniform hardened depth along the peripheral surface of the cam. Moreover, inductive heating in the foregoing manner does not enable any selectivity with respect to varying the inductor coupling with the cam to compensate for profile changes, mass geometry variations and energy density changes in connection with achieving a desired depth of heating. Further, while both the power to the inductor and the rotational speed of the cam shaft can be varied, these capabilities, in connection with an encircling inductor, do not lend to achieving the desired uniform depth of heating and hardening of the cam. Still further, while uniform heating and depth of hardening might be achieved through the use of multiple or split type inductors (with contacts) which in combination provide an interior contour corresponding to the cam profile, such an arrangement would result in a high power requirement, particularly in light of the large surface areas of the cams of diesel cam shafts and this, together with the plural or split inductor requirement would lend to undesirably high production and operating costs. Moreover, production costs would be unacceptably increased by the fact that each different cam profile would require a separate inductor configuration. Still further, the multiple or split inductor arrangement providing an internal bore corresponding to the cam profile would not lend to compensating for large variations in circumferential thermal mass situations with respect, for example, to the basic shaft problem referred to hereinabove, and would not lend to selectively increasing the heated or hardened depth in a particular area of the cam profile where for example, there are higher applied stresses during use of the cam. A further problem with prior heating methods is the possibility of stray heating of adjacent cam surfaces.